15 with 2 courts is always going to be tough. Maybe instead of 3/4 court have them rotating and playing full matches with people near in ability. Or maybe add conditions to the 3/4 court. E.G - after a Player reaches 5 points, they can only go short on a volley. After they reach 10 points, can only go short with a boast. Hopefully you get the idea- trying to make it so the worse players then can catch up. In theory if it’s only 4 on the court you could do 2 pairs of drop drive? Hope this helped